The present invention relates to conversion coating compounds and processes for applying conversion coating to aluminum and aluminum alloys. In particular, the conversion coating compounds disclosed herein enable non-chromate coatings having self-healing properties.
Aluminum, and its alloys, are a common material used in a variety of applications due to their specific strength as compared to other alloys. Unfortunately, numerous aluminum alloys have high negative standard reduction potentials, lending them a tendency to oxidize and corrode. Therefore, conversion coatings are applied to aluminum surfaces to provide protection against corrosion.
Conversion coatings describe a surface film formed by a reaction in which a portion of the base metal is converted to a component of the film. As a result of this reaction and conversion, the film becomes an integral part of the metal surface, exhibiting excellent adhesive properties.
Conversion coatings are generally of two types, chemical and electrolytic. With electrolytic conversion coatings, a metal substrate is immersed in a chemical bath and an electric current is passed through the metal component and the chemical bath to form a conversion coating on the surface of the metal. A chemical conversion solution produces coatings entirely through chemical energy, without assistance from an externally applied electric potential.
When treating aluminum with a chemical conversion coating, the chemical conversion coating solution must include an active agent capable of reacting with both the aluminum substrate and an aluminum oxide surface film that forms whenever oxygen reacts with an aluminum surface. Also required is an agent capable of forming an oxide coating on the surface of the aluminum. For example, an oxidizing agent may assist in forming an oxide coating capable of forming an insoluble compound with aluminum or other ions, or an agent may promote coating formation by a controlled hydrolysis reaction.
Several types of chemical conversion coatings have been developed using such chemical compounds as alkaline oxide, crystalline phosphate, amorphous phosphate, chromate, and boehmite. However, the most widely used active agent in conversion coatings is chromate. Applied in acid solutions, chromate coatings provide effective corrosion resistance and self-healing capacity.
In preventing corrosion through chromate conversion coating of aluminum, an aluminum surface is dipped in deionized water to form a gel layer. The gel layer contains aluminum ions in the trivalent [Al (III)] state. Subsequently, chromate ions in the hexavalent [Cr (VI)] and trivalent [Cr (III)] state replace some of the Al (III) ions in the gel layer. Trivalent Cr (III) ions are believed to allow for hardening of the gel layer and hexavalent Cr (VI) ions are believed to promote self-healing by migrating to active corrosion sites. As such, trivalent Cr (III) ions and hexavalent Cr (VI) ions provide two complementary functions with the net effect of a conversion coating characterized by a hard gel and self-healing.
A self-healing capacity is a known characteristic of chromate coatings. Self-healing is in essence a dynamic repair of newly created breaks or defects in the protective film created by the chemical conversion process. The exact mechanism of self-healing is currently not well understood. While not limiting the present invention to a particular theory of operation, the mechanism for self-healing is currently understood to involve migration of hexavalent Cr (VI) ions from a reservoir in the conversion coating to a distant active exposed site to subsequently inhibit corrosion. This phenomenon is evidenced, for example, by the minimal corrosion that occurs after salt-spray testing even when the sample has a scribe mark through the coating to the metal alloy substrate.
However, solutions containing chromium ions in the hexavalent state have been determined to be carcinogenic. The U.S. Environmental Protection Agency (EPA) has included chromium to the list of toxic chemicals for xe2x80x98voluntaryxe2x80x99 replacement and promulgated strict waste disposal standards to curtail the use of chromium. Strict waste disposal standards and chromium""s listing as a toxic chemical have created a need for alternative chemical conversion coating compounds that do not contain the Cr (VI) ion while preserving effective corrosion inhibition and self-repair.
Some chromate-replacement coatings have been developed to avoid the problems associated with chromate coatings. For example, one method for coating aluminum surfaces uses different metals such as selenium, tellurium, titanium, boron, calcium, cobalt, copper, iron, magnesium, nickel, tin, or zirconium. The method includes forming a hydrated oxide film on the aluminum surface (AlOOH.nH2O) and treating the oxide film with alcoholates of any of these metals. However, the method suffers from serious problems. It is contended that the use of the method results in a coating inferior to chromium conversion coatings because such coating lacks the complementary functions of hexavalent and trivalent ions. The complementary functions provide for hardness of the aluminum gel and self-healing properties. Additionally, the coatings produced by this method are not designed to provide for dynamic repair of newly created corrosion sites or breaks in the conversion coating layer.
Another method currently available for providing chromium-free coating for aluminum includes immersing the aluminum surface in boiling deionized water to form a gel layer, boehmite, on the aluminum surface. The coating layer is then exposed to an aqueous solution of cerium salts to form cerium oxides within the gel layer. However, the resulting cerium coating suffers from various problems. Cerium ions in the trivalent [Ce (III)] state are not known to migrate to active corrosion sites or breaks, unlike Cr (VI) ions. Because Ce (III) ions are incapable of mimicking Cr (VI) ions, Ce (III) ions are unlikely to provide self-healing in any defects in the coating layer. Cerium coating thus provides less corrosion protection than chromate coatings.
Consequently, in light of the problems associated with presently available aluminum chromate-replacement coatings, there exists an unfulfilled need for aluminum non-chromate coatings that mimic chromate coating interactions with outside environment. In particular, there is a need for non-chromate coatings that provide hardness for the coating and dynamic repair at newly created active corrosion sites, breaks or defects in the coating layer. The invention reported herein fulfills this need.
An object of the present invention is to provide for non-carcinogenic conversion coatings to replace current carcinogenic chromate coatings.
Another object of the present invention is to provide for non-chromate coatings that mimic chromate conversion coating characteristics.
Another object of the present invention is to provide for a non-chromate coating that provides a combination of self-healing and durable coating features.
Yet another object of the present invention is to provide for a hard, wear-resistant, corrosion-resistant, and paint adherent surface coating to replace current chromate coatings for aluminum and aluminum alloys.
A related object of the present invention is to provide processes for applying chromate-replacement coatings to aluminum and aluminum surfaces.
A further object of the present invention is to provide for non-chromate conversion coatings that use a wide variety of substrate metals.
Another object of the present invention is to provide for ionic coating solutions enabling specific ions to replace aluminum ions in the gel layer on the aluminum surface.
A related object of the present invention is to provide for an ionic coating solution comprising a combination of at least two different ions, with the exception of solutions comprising only manganate ions.
Yet another object of the present invention is to provide for non-chromate conversion coatings comprising hexavalent and trivalent ions.
A more specific object of the present invention is to enable a non-chromate ionic coating for aluminum having ions that migrate to newly created corrosion sites or breaks in the coating layer.
The above-listed objects are met or exceeded by the present invention. The present invention enables the production and use of chromate-replacement coatings on aluminum and aluminum alloys to provide corrosion resistance and dynamic active repair of any newly created corrosion sites. The present invention provides for both trivalent and hexavalent metallic ions to replace aluminum ions in a gel layer on an aluminum surface. It is contended that such substitution of aluminum ions in the gel by hexavalent and trivalent ions from metallic solutions enables such metallic coatings to act like chromate coatings.
A preferred process for coating an aluminum surface includes cleaning the aluminum surface for removing contaminants, and then immersing the aluminum surface in deionized water. A gel layer forms on the aluminum surface by the reaction of aluminum with water. The gel layer is initially amorphous and contains trivalent Al (III) ions. Thereafter, the aluminum surface is treated with any one of the disclosed chromate-replacement conversion coatings.
Prior art solutions suggested the use of covalent bonding coating solutions. The present invention suggests the use of ionic coating solutions. Prior art solutions suggested the use of a single metal for coating aluminum. The present invention provides for the use of at least two metals, with the exception of manganese, which can be used exclusively. Prior art solutions suggested coating aluminum by first forming a porous gel layer, which contains aluminum ions, on the aluminum surface and plugging the pores of the layer with different metals. The present invention suggests forming a gel layer. Prior art solutions suggested using certain metallic compounds that react with the aluminum ions in the gel layer. The present invention suggests partially replacing the aluminum ions in the gel layer. Prior art solutions suggested the use of metallic ions having several oxidation states. The present invention suggests the use of metallic ions having (III) and (VI) oxidation states. Prior art solutions suggested the use of metallic coatings for aluminum surfaces that were not designed to provide for self-healing. The present invention discloses non-chromate coatings for aluminum that provide for self-healing.